def card_found(list_of_cards):
    list_of_cards.sort()
    old = 0
    for x in list_of_cards:
        if x-old == 2:
            return x-1
        old = x
    return x+1

assert card_found([1, 2, 3, 4]) == 5
assert card_found([4, 1, 7, 8, 3, 5, 9, 10, 6]) == 2
assert card_found([1, 2, 5, 4]) == 3